This invention relates to electrochemical power cells. More particularly, it relates to certain cell designs that minimize the hazards resulting from charging or forced discharging.
Although electrochemical power cells have been known and used extensively for many years, they still have many deficiencies. One of the significant limitations of many cells is a safety hazard when the cell is either force discharged or charged. By forced discharge is meant that an external current is forced through the cell from an external power supply having its negative terminal connected to the positive terminal of the cell and vice versa. By charge is meant an external current is forced through the cell from an external power source having its positive electrode connected to the positive electrode of the cell and vice versa. By cell is meant a single indivisible electrochemical couple consisting of a cathode anode and electrolyte. And by battery is meant an arrangement of more than one cell into an operating multicell unit.
Many cells tend to explode when they are either charged or force discharged. In batteries with low power and energy densities, or cells with small capacities, the hazards are relatively small even if an explosion occurs. However, with large batteries and particularly with new high energy density batteries such as the lithium anode based batteries, explosions can be life endangering events.
It is generally believed that the principal cause of explosions in batteries upon charging or forced discharge is an exothermic reaction due to the reaction of chemical species which were not originally in the battery but were created by electrolysis, or by the distribution of chemicals such as the plating of the anode onto the cathode.
Electrolysis is a well known electrochemical phenomenon. Briefly stated, electrolysis is the decomposition or change in oxidation state of chemicals in liquid form or in solution caused by passing an electrical current through the liquid. As an example, in aqueous electrolyte cells, sufficient external current forced through the cell causes the decomposition of water into hydrogen and oxygen. The hydrogen and oxygen recombine explosively under a veriety of conditions, such as high temperature, spark or the presence of a catalyst. In the case of non-aqueous electrolytes such as those used in lithium cells, there are other chemicals that recombine explosively. For example, in cells where lithium chloride salts are present, electrolysis liberates chlorine gas and elemental lithium which recombine explosively. In the lithium-sulfur dioxide cell, sulfur and oxygen are liberated, both of which combine explosively with lithium under certain conditions. In the highly advanced lithium-oxyhalide cells, such as the thionyl chloride and sulfuryl chloride cells; lithium, sulfur, chlorine and perhaps other unidentified species are liberated by electrolysis and may recombine explosively. If the charging or forced discharging current is sufficiently low, the voltage differential created across the cell may be below the threshold at which electrolysis occurs. In this case, electrolysis does not occur but plating of anode material on to the cathode for fixed discharge and visa versa for charging does occur. And with small particles of anode and cathode material in intimate contact with each other, explosions may occur, particularly if the cell is also exposed to high temperatures. In lithium oxyhalide cells, molten lithium in contact with the oxyhalide usually results in an explosion. Indeed, the lack of a reliable, cost efficient solution to these problems has been a major reason that the lithium-oxyhalide cells have not succeeded commercially.